Medication or other injectable liquid in a syringe or other fluid handling device is frequently administered to a patient through an injection site of an intravenous set or catheter which is in fluid communication with a patient's vascular system. Injection sites or ports are covered by pierceable septums or pre-slit septums and are known in the art and sometimes referred to as "PRN" from the Latin pro re nata meaning "as the need arises." Pierceable septums are usually accessed by use of a sharpened cannula, such as a hypodermic needle which is used to pierce the septum and establish fluid communication with the catheter or I.V. set. There is a trend toward needleless I.V. systems which do not require a pointed cannula to pierce the injection site of an I.V. set. These systems have injection sites covered by a pre-slit septum which can be accessed with a blunt cannula. In some instances medication or other injectable liquid is administered to the I.V. set or catheter in a very short period of time such as one minute. In other instances, it is desirable to maintain the connection between the syringe or other fluid handling device for longer periods of time such as between two and seventy-two hours. Cannula, blunt or sharp, intended for longer term connection with the I.V. set or catheter are sometimes provided with some sort of retention structure to prevent the inadvertent disconnection of the cannula and the I.V. site. In the art, there are cannulas with levers which snap over portions of the I.V. site when the cannula enters the I.V. site. Removal requires the intentional flexure of the levers to unlock the fluid handling device from the I.V. site. Other prior art devices include shielded cannula with structure in the shield and cooperating structure in the cannula shield and on the I.V. site to provide for retention of the cannula in the septum of the I.V. site. Such structures included bayonet slots in the shield which engage pins on the I.V. site. With this structure, the operator is required to twist the shielded cannula after insertion of the cannula in the septum. Shielded cannula having internal threads in the shield are popular and intuitive to use. They can also shield the I.V. site during connection therewith to protect it from inadvertent contamination.
Threaded elements on the exterior of an I.V. site and on the interior of a cannula shield are tolerance sensitive. For example, shields, which have threads with peaks describing a diameter which is smaller than the root diameter of the thread on the I.V. site, may be impossible or difficult to engage and disengage. On the other hand, threads that are loose will engage easily and smoothly but may, in certain circumstances, tend to come loose. Such an occurrence will disconnect the medication or injectable liquid from the patient frustrating the patient's plan of therapy. Accordingly, although a threaded shielded cannula is a preferred device for connecting cannula to I.V. sites, there is still a need for a threaded shielded cannula which is less sensitive to manufacturing tolerances and provides high resistance to removal during the use of the product, and more preferably a higher resistance to removal than the resistance to insertion.